Microtremor observations in Almeria, Spain

Recent destructive earthquakes have shown that, generally, in unconsolidated soils and sedimentary deposits soil movement can be amplified, in a frequency band increasing shaking duration, and also large differences at the level of damage for relatively short distances. In order to evaluate site response in Almeria city, there have been made microtremor measurements in many points of the city in a 200m x 200m grid, using a 3 components sismometer of high sensibility, with a natural period of 1 sec. For checking the periods and amplitude stability of microtremors, we have realized stationarity measurements in hard, middle-hard and soft soil in a range of 24 hours, measuring them at every hour for 3 minutes, observing that although periods present a good stationarity, amplitude varies significantly. The application of Nakamura's method has allowed us determine the predominant period at every point. The range period oscillates between 0.06s for very hard soil (pliocene rocks and Pleistocene materials) and 1.4s for very soft soil (Holocene matepal and coastal sand). A good correlation is observed between the map of predominant periods values distribution and surface geological conditions of the soil.